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The gas phase infrared spectra of the hydrated hydronium cluster ions H3O+⋅(H2O)n(n=1, 2, 3) have been observed from 3550 to 3800 cm−1. The new spectroscopic method developed for this study is a two color laser scheme consisting of a tunable cw infrared laser with 0.5 cm−1 resolution used to excite the O–H stretching vibrations and a cw CO2 laser that dissociates the vibrationally excited cluster ion through a multiphoton process. The apparatus is a tandem mass spectrometer with a radio frequency ion trap that utilizes the following scheme: the cluster ion to be studied is first mass selected; spectroscopic interrogation then occurs in the radio frequency ion trap; finally, a fragment ion is selected and detected using ion counting techniques. The vibrational spectra obtained in this manner are compared with that taken previously using a weakly bound H2 ‘‘messenger.’’ A spectrum of H7 O+3 taken using a neon messenger is also presented. Ab initio structure and frequency predictions by Remington and Schaefer are compared with the experimental results.

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The hydrogen-bonded and free OH stretch modes of Cl -(H 2 O) n (n ) 1-5) have been observed by vibrational predissociation spectroscopy in the 2.6-3.2 µm region. Besides demonstrating that all clusters form strong ionic hydrogen bonds, the spectra provide clear evidence of water-water hydrogen-bonding networks in n ) 4 and n ) 5, with the broad spectrum of n ) 5 resembling that of large neutral water clusters. No waterwater hydrogen bonding is seen in n ) 2 and n ) 3, but these clusters appear to be solvated asymmetrically. While the data suggest that Clion is solvated on the surface of water clusters, there are discrepancies between the observed spectra and ab initio predictions. This disagreement may stem from either zero-point motion or high cluster temperature, which tend to disrupt hydrogen bonding among the waters.

Honing in on HONO 2 Modeling air pollution requires knowledge of all the interrelated reactions occurring in the atmosphere. Among the most significant is the formation of nitric acid (HONO 2 ) from OH and NO 2 radicals. One sticking point in the study of this reaction has been the uncertainty in how often radicals link through an O-O rather than an O-N bond. Mollner et al. (p. 646 ) measured the partitioning coefficient, as well as the overall consumption rate of the radicals, with an array of highly sensitive spectroscopic techniques in the laboratory. The measurements yielded a well-defined rate constant for nitric acid formation, which was applied to the prediction of ozone levels in atmospheric simulations of the Los Angeles basin.